1. Field of the Invention
This invention relates generally to strong, porous, thin improved electrical connectors and supports for tubular solid oxide electrolyte fuel cells in a fuel cell generator.
2. Background Information
Square pitched, series-parallel interconnection of solid oxide fuel cells is well known, and taught in U.S. Pat. Nos. 4,490,444 and 4,833,045 (Isenberg and Pollack et al., respectively). The fuel cells used usually contain a self-supported air electrode tube, where the air electrode is covered over about 300 degrees by a solid electrolyte film. Thus, there is a 60 degree wide axial strip down the length of the cell. This remaining 60 degrees of air electrode surface is covered by an interconnection strip, usually a lanthanum-chromite. As a top layer, fuel electrode covers the solid electrolyte over about 280 degrees of the electrolyte surface.
These cylindrical cells are placed in a square pitch, series-parallel connected array, wherein the air electrode of one cell is connected to the fuel electrode of the adjacent series-connected cell by a plated interconnection strip and a strip of 80% to 95% porous sintered nickel felt, which is about 0.1 inch (0.25 cm) thick. Other nickel felts provide parallel connections between the fuel electrodes of adjacent cells. The series path is essential for the generation of a practical DC stack voltage. The parallel connections provide paths by which the current can circumnegotiate any defective open circuit cells. Fuel flows axially in the passages forced between the groups of cells. This has been the standard design for over fifteen years.
In this standard design, the primary subassemblies from which a solid oxide fuel cell generator is formed are cell bundles. Presently, bundles contain twenty-four cells on a 8xc3x973 cell matrix. Eight cells are series connected to form one row of a three-row bundle. The three rows are connected in parallel through the connection of each cells in the row with the adjacent cell in the next row. Between the nickel plated lanthanum-chromite interconnection strip of one cell and the nickel fuel electrode of the next cell in a row, any two cells are presently series connected by a nickel felt of a rectangular cross-section (approximately 6.3 mmxc3x976.3 mm). These felts are pressed to a thickness of between about 0.1 inch (0.25 cm) and about 0.25 inch (0.63 cm) and are initially about 80% to 95% porous. Parallel connection is also currently accomplished by similar felt strips. In this case, the felts connect the fuel electrodes of adjacent cells. Along the length of a cell, eight felts of 185-mm length are used to form a series connection, and four felts of 185-mm length are used to accomplish a parallel connection. A total of 280 felt strips are used per bundle. This means of electrical connection is effective; however, it is costly in terms of materials and is very labor intensive. Furthermore, this arrangement is not conducive to automation.
Improvements to this standard design have been suggested. Reichner, in U.S. Pat. No. 4,876,163, disclosed spiral or folded row connections within a cylindrical generator, using U-shaped connections. This design, however, substantially decreases gaseous fuel flow between the outer electrodes of the cells. U.S. Pat. No. 5,273,838 (Draper et al.) eliminated one nickel felt connector from each group of four cells, where alternate cells of a first row had no electrical connection of their interconnections to cells in an adjacent row. This design helped to eliminate the potential for bowing when using newer, longer one meter cells. This design may, however, decrease the overall strength of the twenty-four cell subassemblies.
In an attempt to simplify generator design and reduce assembling costs, DiCroce et al., in U.S. Pat. No. 5,258,240, taught a thick, flat-backed, porous metal fiber felt connector strip, having a crown portion of metallic fiber felt conforming to the surface of its contacting fuel cell. These porous felt connectors could be used as a series of thin strips across a small part of the fuel cell length, or as a porous sheet extending along the entire axial length of the fuel cells. In order to provide structural integrity, since there are no side connections, a plurality of cells would have to be laminated to provide a thickness of 0.125 inch (0.62 cm), thereby reducing porosity to about 5 to 10%. The strips could also be made of a solid nickel foil or a composite of foil and porous felt; they could also have two opposing fuel cell conforming surfaces, as shown in FIG. 3 of that patent.
The use of fibrous felts still allowed potential densification during prolonged use. Additionally, it was difficult to fashion such felts to exact dimensions, and the felts retailed a springiness. Conversely, the use of foils did not provide adequate strength, and prevented the required infiltration of the bundle with hot air during the drying process, which is an important feature of bundle manufacture.
What is needed is a highly porous nickel support made of a single piece, across a horizontal fuel row, to conform to and support all contacting fuel cells, as well as to connect all contacting fuel cells electrically. The support must be strong, but it must also be possible to introduce the desired flexibility by selection of an appropriate form or shape.
Therefore, it is a main object of this invention to provide a thin, strong, porous electrical connector and support for tubular solid oxide electrolyte fuel cells in a fuel cell generator.
It is also a main object of this invention to provide an improved method of connecting and supporting fuel cells in a fuel cell generator.
These and other objects of the invention are accomplished by providing a solid oxide fuel cell assembly comprising rows of fuel cells, each having an outer interconnection and an outer electrode, disposed next to each other with corrugated electrically conducting metal mesh between each row of cells, the corrugated mesh having a top crown portion and a bottom shoulder portion, where the crown portion contacts the outer interconnections of the fuel cells in a first row, and the shoulder portion contacts the outer electrodes of the fuel cells in a second row, said mesh electrically connecting each row of fuel cells, and where there are no metal felt connections between any fuel cells.
The invention also comprises a solid oxide fuel cell assembly comprising a first row of spaced apart, axially elongated tubular fuel cells, each containing an outer electrode and an outer interconnection; a second row of spaced apart, axially elongated tubular fuel cells, each containing an outer electrode and an outer interconnection, the second row being spaced apart from the first row, where all the outer interconnections of the first row fuel cells face all the outer electrodes of the second row fuel cells; and an electrically conducting connector support for the fuel cells, extending between and contacting the first row and the second row of fuel cells, where the connector support consists of an expanded mesh more than about 60% porous and having a thickness between 0.025 cm (0.01 inch) and 0.076 cm (0.03 inch), said connector support having a corrugated structure with a series of top crowns connected to bottom shoulder sections, where the crowns and shoulder sections conform to the shape of their contacting tubular fuel cells, where each shoulder section is connected to its adjacent shoulder section, where all of the top crowns contact the interconnections of the first row fuel cells and all of the shoulder sections contact the outer electrodes of the second row fuel cells, and where there are no metal felt connections between any fuel cells. In some embodiments, the sides of the crown portion and the connection between shoulder portions will also be corrugated, so as to provide additional flexibility or springiness to the assembly. Preferably, the connector support is made of nickel.
The invention also comprises a method of manufacturing a solid oxide fuel cell assembly, comprising the steps of: (1) providing a first and second row of spaced apart, axially elongated tubular fuel cells, the second row being spaced apart from the first row, each fuel cell containing an outer electrode and an outer interconnection, where all the outer interconnections of the first row fuel cells face all the outer electrodes of the second row fuel cells; (2) providing a flat sheet of expanded nickel mesh having a porosity over about 60% and a thickness between 0.025 cm and 0.076 cm; (3) heating the expanded nickel mesh to make it formable; (4) forming the heated, flat, expanded nickel mesh sheet into a corrugated structure with a series of top crowns connected to the bottom shoulder sections to provide a connector support, where the forming returns the strength and temper to the nickel mesh, and where the crowns and shoulder sections will conform to the shape of the tubular fuel cells of the first and second rows of fuel cells; (5) adding an organic adhesive mixed with nickel powder to the crowns and shoulder portions of the corrugated nickel mesh connector support; (6) disposing the adhesive-containing corrugated connector support between the first and second rows of fuel cells, such that all of the connector support top crowns contact and adhere to the interconnections of the first row fuel cells and all of the connector support shoulder sections contact and adhere to the outer electrodes of the second row fuel cells; (7) drying the adhesive by passing hot air through the porous connector support; and (8) sintering the fuel cell assembly to vaporize the organic portion of the adhesive and provide an integral fuel cell assembly. The sintering can be accomplished during manufacture of a cell bundle or during startup of a fuel cell generator containing the assembly. Epoxy resin has been found to vaporize easily and provides no ill effects on the fuel cell components. Additionally, the sections between crown top and shoulder are corrugated for increased flexibility.
This provides a very porous, ultra thin, and extremely tough electrical connector support for tubular solid oxide fuel cells, allowing elimination of hand labor, reducing total parts for a 24-cell bundle by over 80% and cutting production time by 90%, as well as allowing automated assembly.